1. Field of The Invention
The present invention relates to projection television and, more particularly, to a projection lens system useful in a projection television.
2. Description of Related Art
A projection television is a device by which images from red, green, and blue projection CRTs are projected through three associated projection lens systems onto a screen to form an enlarged color picture. In order to reduce the thickness or size of a projection television, the projection lens systems employed are required to have a large viewing angle and aperture ratio, as well as good imaging performance.
Many types of projection lens systems have been proposed for satisfying these requirements, and typical examples include: a projection lens system that is solely composed of glass lenses that can be machined with high precision to retain good quality; a system that is solely composed of plastic lenses with a view to reducing the manufacturing cost while attaining a large aperture ratio; and a hybrid system composed of glass lenses and aspherical plastic lenses.
The conventional projection lens systems have their own problems. A problem with the system that is solely composed of glass lenses is that its size and cost will be increased and its marginal performance be greatly impaired if an attempt is made to improve its imaging performance while maintaining a large aperture ratio. The system that is solely composed of plastic lenses having an aspherical surface is suitable for the purpose of attaining a large aperture ratio but the precision of machining that can be attained with plastic lenses is not as high as the value achievable with glass lenses s that this system is not capable of operating with its design performance being fully exhibited. A further problem with plastic lenses is that they experience changes in refractive index and shape in the face of temperature variations and that the resulting change in the focal position will impair their imaging performance.
It is generally held that the lenses through which images from red, green, and blue projection tubes are projected onto a screen in a projection television need not be achromatic. In practice, however, a blue projection tube produces a broad emission spectrum whereas spurious emission spectra are observed in green and red projection tubes. Therefore, non-achromatic projection lenses have a disadvantage in that the imaging performance for the three primary colors is reduced to produce a lower contrast.